1. Field of the Invention
The present invention generally relates to fuel cell systems and, in particular, to fuel cell systems employing improved fuel recirculation at low loads.
2. Description of the Related Art
Electrochemical fuel cells are used to produce energy by converting reactants, namely fuel and oxidant, through the electrochemical reactions that take place within the fuel cell. They do not generate any pollutants and therefore have gained popularity as an attractive alternative to the internal combustion engine. One type of fuel cell that has been used for automotive and other industrial applications because of its low operating temperature is the solid polymer fuel cell. Solid polymer fuel cells employ a membrane electrode assembly (“MEA”) that includes an ion exchange membrane disposed between two electrodes that carry a certain amount of catalyst at their interface with the membrane.
The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). A catalyst is needed to induce the electrochemical reactions within the fuel cell. The electrodes may also comprise a porous, electrically conductive substrate that supports the catalyst layer and that is also employed for purposes of electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer.
During normal operation of a solid polymer fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the ion-exchange membrane, to electrochemically react with the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water as a reaction product.
During normal fuel cell operation water is created on the cathode as a result of fuel oxidation. Some of the water produced at the cathode may pass to the anode side where it can condense, creating water droplets that may block the fuel flow field channels. This may result in insufficient fuel being provided to the active area of the fuel cell. In some fuel cell systems excess fuel is passed through the channels to expel the water droplets. However, venting excess fuel into the atmosphere is undesirable in many instances.
As a result, and in order to minimize the waste that would result from venting the unconsumed reactants, the reactants may be recirculated via a recirculation loop. For example, the recirculated reactant may be merged directly with the incoming fresh reactant stream, thereby humidifying the incoming fresh reactant stream with the accumulated product water and avoiding the need of a separate humidifier.
A pump or a blower is typically used to move the reactant through the recirculation loop. However, these devices increase the parasitic load on the fuel cell and present other disadvantages in weight, cost, and reliability. Vacuum ejectors (jet pumps) have also been employed to effect recirculation. However, an ejector sized to supply the needed inlet flow rate to the fuel cell stack during periods of maximum-load typically employs a nozzle that is too large to recirculate the fuel during periods of low-load (e.g., idle periods). Thus, at low loads the fuel feed pressure is too low to ensure satisfactory recirculation.
This problem has been addressed by modifying the construction of the jet pump, or through control strategies to ensure satisfactory fuel recirculation. For example, a multiple jet ejector assembly is described in U.S. Application Publication No. 2005/0064255 that includes a common suction chamber, a low-flow nozzle and a low-flow diffuser, and a high-flow nozzle and a high-flow diffuser. The low-flow nozzle and diffuser are configured to entrain the recirculated flow at low loads, while the high-flow nozzle and diffuser are configured for high-load conditions. A separate nozzle and diffuser may be provided for ultra-low flows, as needed during idle conditions.
Alternatively, U.S. Pat. No. 5,441,821 describes a control strategy involving a regulated pressure control valve capable of maintaining a relatively uniform inlet fuel stream pressure over a range of operating conditions under different loads, and consequently maintaining a relatively uniform outlet fuel stream recirculation ratio. The pressure control valve is also regulated to maintain a balance between the pressure of the inlet fuel stream and the pressure of the inlet oxidant stream.
While various advances have been made in this field, there remains a need for a less costly, less complex and/or more efficient approach to operating a fuel cell system with improved fuel recirculation at idle conditions, while allowing the removal of water droplets from the anode side. The present invention addresses this issue and provides further related advantages.